During the processing of products and general cargo, especially during packaging, the products or cargo are often stacked in several layers one above the other. Intermediate layers are often inserted between the several layers of products or cargo. These intermediate layers are usually two-dimensional objects. The term two-dimensional object refers to layers of material, whereby the thickness of the material is negligible in comparison to the length and width of the material. The term two-dimensional object especially refers to intermediate layers made from film, paper or any other material with similar properties, especially to any flexible material with a negligible thickness. Such two-dimensional objects or plates may for example be formed by cardboard intermediate layers, plastic intermediate layers, twin wall sheets, sheets of corrugated cardboard, etc., either made from cardboard or plastic material. It is often problematic to raise such two-dimensional objects or stacked plates individually, because the objects or plates tend to stick to each other. The unwanted sticking effects are especially due to adhesion forces or mechanical fiber entanglement or low pressure attachment. These attachment forces have to be reduced or rendered ineffective during the lifting of the individual sheets or plates from a stack. Faster cycle times of gripping and lifting devices lead to more pronounced effects of these unwanted adhesion phenomena. To avoid or at least reduce the sticking of the plates, suitable brush elements can be used. When the uppermost plate is lifted, the plate under is retained by these brush elements. In practice it has been found that lifting and bending the lateral areas of the uppermost plate can be used as an effective measure to prevent the unwanted effect that the plate below is also dragged along. Another measure to avoid the problems mentioned above may be the use of frame magazines with a pre-separation of the plates.
EP 0 639 519 A1 shows a separation device for sheet or plate material with a plurality of vertically-oriented vacuum grippers. The vacuum grippers contact the uppermost plate for lifting. A peripherally arranged vacuum gripper is swivel-mounted and/or the support of the peripherally arranged vacuum gripper is displaceable in a horizontal direction. The peripherally arranged vacuum gripper can bend the peripheral area of the plate before the whole device is lifted.
A device for separating flexible plate-like objects such as metal plates by means of suction carriers, which are also called suction grippers, is also known from EP 1 215 148 A1. The majority of vacuum grippers are associated with peripherally located so-called separation suckers. The separation suckers are pivotable about an articulated joint by a small amount compared to the suction carriers working in a vertical orientation. The separation suckers can lift the peripheral areas of the uppermost plate from the underlying plate and can bend this peripheral area upwards.
EP 1864922 B1 discloses an apparatus and method for lifting an uppermost plastic plate or sheet from a stack of plates by means of suction grippers. The suction grippers are put onto the uppermost plastic plate. Then vacuum is applied and the uppermost plate is lifted from the stack. To prevent a sticking of the uppermost plate to the plate below, the lifted plastic plate is initially lifted parallel to the stack and then subjected to a two-way bending. The disclosed device for lifting the uppermost plastic plate from the plate stack includes a vertically movable carrier with suction grippers mounted thereon. The suction grippers can be connected to a vacuum source. On the carrier the suction grippers are arranged in pairs of external and internal suction grippers. The inner suction grippers can be temporarily connected to a vacuum source. The outer suction grippers can be temporarily and alternately connected to a vacuum source and a compressed air supply via a switching valve.